Constant-load spring-loaded stationary starting switch for electric motors



Oct. 23, 1956 1 y GREENHUT 2,768,260.

CONSTANT-LOAD SPRING-LOADED STATIONARY STARTING SWITCH FOR ELECTRICMOTORS Filed April 2, 1955 2 Sheets-Shea. 1

\ new INVENTOR. JOSEPH GREENHUT sygwxm ATTORNEYS Dct.

Filed April 2, 1955 jllllll F SYNGHRONOUS SPEED OF SYNGHRONOUS SPEED 23,1956 J. GREENHUT CONSTANTLOAD SPRING-LOADED STATIONARY STARTING SWITCHFOR ELECTRIC MOTORS 2 Sheets-Shee' 2 FIG. 6

43 o .2 .4 .s .8 L0 |.2 L4 L6 i DEFLECTION h CONE HEIGHT FIG, 8/

50 I00 I50 200 250 300550 400450500 0F FULL LOAD TORQUE OF FULL LOADTORQUE F l6. l0

INVENTOR. JOSEPH GREENHUT lax km ATTORNEYS O 50 I00 I50 200 250 300 350400 450 500 OF FULL LOAD TORQUE United States Patent CONSTANT-LOADSPRING-LOADED STATION- ARY STARTING SWITCH FOR ELECTRIC MOTORS JosephGreenhut, University Heights, Ghio Application April 2, 1953, Serial No.346,372

7 Claims. (Cl. 200--80) This invention relates to the starting switchesof electrical motors and more specifically to the stationary switchingelement of the starting circuit of a single phase induction motor of thesplit-phase type.

In split-phase motors it is necessary to provide an auxiliary orstarting winding which will produce the necessary torque characteristicsto cause initial rotation of the rotor and will also bring the rotor upto a speed at which the torque developed by the main winding alone isgreat enough to overcome the load on the motor. In resistance-start,reactor-start and capacitor-start split-phase motors, a switching deviceis employed which maintains the starting winding in the main windingcircuit until the rotor has reached a predetermined speed.

Such switching devices may be of various forms and designs and generallyprovide for a resilient stationary switch or contact mechanism which isactuated by a springloaded speed-responsive centrifugal device.Reference is made to my United States Patent No. 2,616,682, relating toan improved centifugal actuating unit, for a more detailed descriptionof such starting devices.

There are, of course, many correlated factors which must be taken intoconsideration in designing such switch ing devices for a particularmotor design. Among these factors, is the tension or load characteristicon the centrifugal or rotary unit which is a resultant of the combinedeffect of the spring characteristics of both the stationary switch andthe rotary actuating unit.

It would be expected that the operating characteristics of a startingswitch would be consistent and uniform after such a switch had beendeveloped and tested in a motor of particular design. This would be so,if all the factors upon which the switch design and motor design werebased, remained constant. However, large scale production methods mustallow for manufacturing tolerances which ordinarily are kept Withinlimits which do not affect product performance.

It has been found that the cumulative eifect of dimensionalmanufacturing tolerances in the production of electric motors is oftenreflected in unsatisfactory and unreliable starting switch operatingcharacteristics. It has been established that the load characteristicson the rotary unit, specifically mentioned in the next precedingparagraph, are extremely sensitive to variations in the spacing betweenthe stationary switch and the rotary unit, even though such dimensionalvariations are restricted to the range of practical manufacturingtolerances.

Thus, one of the factors, which should be fixed or constant in order toobtain uniformly consistent starting Switch performance, is, instead,variable and cannot be controlled within the limits of mass productionmanufacturing tolerances.

It is the primary object of my invention to provide a starting switchwhich will, within practicable limits, be unaffected in its performancecharacteristics by dimensional spacing variations of the characterabove-descrbed.

Another object of my invention is to provide a method for constructingstarting switches to obtain consistency of performance characteristicsin spite of dimensional spacing variations.

Other objects and advantages of my invention will be apparent during thecourse of the following description.

In the accompanying drawings, forming a part of this specification, andin which like numerals are employed to designate like parts throughoutthe same,

Fig. l is a side elevation of an electric motor embodying the featuresof my invention, portions thereof being broken away to show details ofthe starting switch.

Fig. 2 is a plan view of the stationary switch viewed as indicated bythe line 2-2 on Fig. 1.

Fig. 3 is a fragmentary transverse cross-sectional view taken asindicated by line 3-3 of Fig. 2.

Fig. 4 is a transverse cross-sectional view of the stationary switchtaken on line 44 of Fig. 2.

Fig. 5 is a side elevation of the switch unit shown in Fig. 1.

Fig. 6 is a graph showing the load-deflection characteristics of thespring element of the stationary switch shown in Figs. 1-5.

Figs. 7, 8 and 9 are graphs showing the speed-torque curves of splitphase motors under varied conditions of manufacturing tolerance, and

Fig. 10, is a fragmentary side elevation of a modified form ofsecurernent for the stationary switch illustrated in Figs. l5.

Referring more particularly to the drawings, I have shown in Fig. l arepresentative electric motor 11 including a motor casing or housing 12,a stator 13, a rotor 14 and a rotatable shaft 15. These elements are ofconventional design and form no part of the present invention.

FiXedly secured to the shaft 15, in abutment with an annular shoulder 16thereof, is a speed-responsive cen trifugally-energizable switchactuating unit 17 which is commonly and more briefly termed the rotaryunit or rotary. The rotary 17 may be of conventional design andreference is made to my U. S. Patent No. 2,616,682 for a description ofa form of rotary which may be utilized.

Briefly, the rotary unit 17 includes a body member or base plate 18, aresilient toggle plate or spring 19, a plurality of movable Weights 20secured to the toggle plate 19 and a longitudinally movableswitch-engaging shoe 21. The movement of the weights 20, in response tocentrifugal force, causes the shoe 21 to withdraw or retract from thecircuit closing arm of the switch in a manner well known in the art. Itis to be noted that the switch arms of the prior art starting switchesare resilient or spring-pressed so that the starting circuit will beopened when the shoe 21 is retracted. The spring pressure of thestationary switch is opposed to the spring pressure of the rotary unit.The foregoing is disclosed in the above-mentioned patent.

Before proceeding with a description of the novel stationary switchstructure disclosed in the drawings, reference will be made to thegraphs shown in Figs. 7, 8 and 9 of the drawings for a fullerunderstanding of the problem here involved.

In Fig. 7 is shown the speed-torque curve of a representativehigh-torque spilt-phase motor. The curve A is developed for the main orrunning winding of the motor. The curve B is developed for the combinedstarting and main windings. It will be noted that the combined windingsdevelop a much greater torque than the main winding alone until themotor attains about 35 0 of synchronous speed. At higher speed, the mainwinding develops more torque than the combined windings. Thischaracteristic of spilt-lapse motors is well known and is the basis forthe use of speed-responsive switches to open the starting windingcircuit when a predetermined motor speed has been attained. The line Cdesignates the switch operating speed and represents the speed at whichthe switch is operative to open the starting winding circuit thuspermitting the motor to run on the main winding alone. The solid linesindicate the circuits actually utilized in bringing the motor up to fullspeed from a position of rest.

Purely from a maximum torque standpoint, the switch operating speedshould coincide with the crossover point of the curves A and B. As amatter of practice, however, the starting switches are generallydesigned to operate at a slightly lower speed; in this instance, atabout 78% of synchronous speed. If, for any particular design of motorand starting switch, the switches would operate at the speed for whichthey were designed, there would be no particular problem. The fact isthat identical switch assemblies, mounted in several motors of identicalmanufacturing design, will not operate at identical speeds.

I have found that this unexpected and unpredictable variation in switchoperating speeds can generally be traced to variations in thedimensional spacing between the rotary and the stationary switch. Itmust be emphasized that this spacing variation does not represent faultymanufacture, but is entirely within the practicable manufacturingtolerances established.

In Fig. 8, the curves A and B of Fig. 7 have been reproduced toillustrate one of the elfeots on the switch operating speed as aconsequence of spacing variation. In this instance, although theindividual components of the motor were all within the establishedlimits of manufacturing tolerance, the cumulative eflFect of minordimensional variations resulted in a change of plus .045 inch in thespacing between the stationary and rotary units. Thus, instead of beingspaced apart at inches as in Fig. 7, the switch elements were spaced xplus 0.45 inch. This increased spacing resulted in a decreaseddeflection of the stationary switch arm and a corresponding reduction inthe pressure of force exerted by it upon the rotary. The result was thatthe interplay of spring pressure on the rotary was diminished and thecentrifugal forces acting on the rotary had to be augmented to actuatethe rotary. Therefore, as illustrated by line C in Fig. 8, the switchoperating speed increased to about 92% of synchronous speed, and thestarting winding was retained in the circuit overly long.

In Fig. 9, I have again shown the curves A and B and have illustratedthe circuits established as a result of a variation in spacing in theminus direction. In this instance the cumulation of tolerances resultedin a spacing of x minus .045 inch between the stationary and the rotaryelements. The increased deflection of the switch arm created anundesirable and unforeseeable increase in the interplay of spring forceswhich resulted in the lower switch operating speed indicated by line C.The resulting opening of the starter winding circuit at about 63% ofsynchronous speed caused a premature switchover to the main winding onlywith a torque drop of almost 150 percentage points.

Figs. 8 and 9 are thus illustrative of the adverse elfects which arecaused by spacing variations of the character described. To overcomethis difficulty by narrowing the tolerance limits in manufacture would,if otherwise feasible, be prohibitive in cost.

My method of overcoming the difficulty involves no significant increasein cost and consists broadly in maintaining the spring pressure of theswitch assembly substantially constant and independent of spacingvariations of the magnitude which would result from cumulations of themanufacturing tolerances which are ordinarily used in the motorindustry. An embodiment of this method is shown in Figs. l of thedrawings to which further reference will now be made.

The starting winding (not shown) is connected by leads 22, 23 to astationary switch assembly 24. The

assembly 24 includes a dielectric plate or mounting panel 25 which isprovided with a central aperture 26. The panel 25 is secured to themotor housing 12, interiorly thereof, by means of suitable fastenerssuch as rivets 27. The shaft 15 extends through the aperture 26 of thepanel 25 without interference.

Secured to the plate 25, as by rivet 28, is a contact arm 29, which maybe of Phosphor bronze or copper, and which is provided with a contactbutton 30. The lead 22 is connected to the contact arm 29.

A conical disc spring 31, having radially extending circurnferen tiallyspaced ears 32, is mounted on the panel 25 in spaced relationshipthereto. Two of the cars 32 are notched as at 33 and these ears slidablyengage two anchor pins 34 which are fixedly secured to the plate 25. Asbest seen in Fig. 3, the anchor pins 34 are each provided with aV-shaped recess 35 in which the ear 32 is received. The surfaces 36 ofthe notch 35 serve as bearing faces or fulcrum during the flexingmovement of the spring 31 and the notches also provide adequate reliefto permit the slight degree of radial movement which occurs duringflexure of the spring. The marginal portion of the disc spring 31 whichlies intermediate the notched ears 32 may be additionally anchored bymeans of a retaining clip 37 which overlies the spring, as indicated,and is secured to the panel 25 by means of a rivet 38.

The third car 32 has a contact button 39 afiixed to the undersidethereof so that it overlies and is in registry with the contact button30.

A retainer arm 40 is secured to panel 25 by rivet 41 and overlies theear 32 to limit the outward movement thereof, as shown in Fig. 4.

The spring 31 may be stamped out of a cup-shaped disc and includes afrusto-conical ring portion 42 having integral arcuate lever arms 43which project interiorly of the ring 42 and are substantially concentrictherewith. The arms 43 also project outwardly slightly or away from thepanel 25.

The lead 23 is connected to one of the notched ears 32 and theelectrical circuit to the starting winding is thereby closed when thecontact buttons 30 and 39 are in abutment.

As best seen in Figs. 1 and 5, the shoe 21 of the rotary 17 abuts thelever arms 43 of the stationary switch 24. It will, of course, beunderstood that the rotary 17 rotates with the shaft 15. In Fig. l themotor is shown at rest and the spring-pressed shoe 21 bears against thelever arms 43 to maintain the contact button 39 on spring 31 in abutmentwith the contact button 30 on arm 29. The starting circuit is therebyclosed. The position of the various parts at rest is also shown inbroken lines in Figs. 3 and 5.

When the motor is energized, the weights 20 swing outwardly in responseto centrifugal force and at a predetermined specd cause retraction ofthe shoe 21 to the solid position shown in Fig. 5. The load on thespring 31 is thereby reduced and the contacts 30 and 39 separate, asshown in Fig. 4, to open the circuit to the starting winding.

The switch operating speed will remain substantially uniformirrespective of variations in the switch spacing which may occur as aresult of manufacturing tolerances in individual motor assemblies ofidentical design. This improved result is obtained because the spring 31is a constant-load spring and exerts a substantially uniform pressureover a relatively wide range of deflection.

In Fig. 6 this characteristic of the spring is shown graphically by theload-deflection curves. it will be noted thata substantial portion ofthe curve shows steadily increasing deflection at substantiallyconstant-load. Thus the load characteristics of the spring 31 would notbe materially affected by moderate dimensional spacing variations whichwould increase or decrease the deflection of the spring.

By incorporating a constant-load spring in a stationary switch, theswitch operating speed of the rotary remains unaffected by a variationin spacing of the character heretofore described. Dimensional deviationson the order of plus or minus .045 inch, or a total of .090 inch, whichhave heretofore produced the undesirable characteristics illustrated inFigs. 8 and 9, have had no significant effect on switch operating speedswhen the constant-load spring is embodied in the switch assembly.

In Fig. 10, I have shown a modification of the invention in which aguide pin 44, having an enlarged head 45, is utilized to limit themovement of the spring 31, in lieu of the previously described retainerarm 40. In this form of the invention, the ear 32, which carries contactbutton 39, is notched to slidably engage the shank or stem 46 of theguide pin. The head 45 of the pin limits the movement of the ear 32.

The form of stationary switch shown in the drawings is exemplary of anembodiment of my invention. It may be noted at this point, that theconstant-load disc spring lends itself readily to incorporation in astationary switch and I consider its use preferable to other forms ofresilient elements. In using the constant-load disc spring, it isimportant that the spring be elevated slightly above the surface onwhich it is mounted, as by the pins 34, illustrated. This mountingpermits deflection of the spring 31 beyond the abscissa point 1.0 shownon the chart in Fig. 6. If the spring is mounted flush against a flatsurface, the constant-load deflection characteristics are restricted toa much narrower range than is shown in Fig. 6, and the full advantagesof the constant load disc spring are not obtained.

The lever arms 43 may be varied in length and in position to suit thecharacteristics of the switch design or the motor design. The leverarms, of source, produce a mechanical advantage in relation to the ringportion 42 of the spring and produce an effective deflection range forthe spring which is proportional to the basic deflection range of thering portion.

It is to be understood that the form of my invention, herewith shown anddescribed, is to be taken as a preferred example of the same, and thatvarious changes in the shape, size and arrangement of parts may beresorted to, without departing from the spirit of my invention, or thescope of the subjoined claims.

Having thus described my invention I claim:

1. In a motor starter device of the type having a movable switch,resilient means operatively engaging said switch to maintain it inclosed position, and shaft-mounted speed-responsive means engaging saidresilient means to disengage said resilient means from said switch, saidspeed-responsive means being axially spaced from said switch Withinpredetermined dimensional limits defining a spacing range; theimprovement consisting of a resilient element engaging said switch inopposition to said resilient means and yieldably urging said switch toopen position, said element being characterized by a substantiallyconstant load factor within a range of deflection corresponding to saidspacing range.

2. In a motor starter device of the type having a movable switch,resilient means operatively engaging said switch to maintain it inclosed position, and shaft-mounted speed-responsive means engaging saidresilient means to disengage said resilient means from said switch, saidspeed-responsive means being axially spaced from said switch withinpredetermined dimensional limits defining a spacing range; theimprovement consisting of a mounting panel, a fixed contact of saidswitch carried by said panel, a resilient element mounted on said panel,and a movable contact of said switch carried by said resilient element,said resilient element being disposed in opposition to said resilientmeans to yieldably urge said switch to open position, and said elementbeing characterized by a substantially constant load factor within adeflection range corresponding to said spacing range.

3. In a motor starter device of the type having a movable switch,resilient means operatively engaging said switch to maintain it in itsclosed position, and shaftmounted speed-responsive means engaging saidresilient means to disengage said resilient means from said switch, saidspeed-responsive means being axially spaced from said switch withinpredetermined dimensional limits defining a spacing range; theimprovement consisting of a conical disc spring engaging said switch inopposition to said resilient means to yieldably urge said switch to openposition, said spring being characterized by a substantially constantload factor within a range of deflection corresponding to said spacingrange.

4. In a motor starter device of the type having a movable switch,resilient means operatively engaging said switch to maintain it inclosed position, and shaft-mounted speed-responsive means engaging saidresilient means to disengage said resilient means from said switch, saidspeed-responsive means being spaced axially from said switch withinpredetermined dimensional limits defining a spacing range; theimprovement consisting of a mounting panel supporting said switch, and aconical disc spring carried by said mounting panel in elevated spacedrelationship thereto and engaging said switch in opposition to saidresilient means to yieldably urge said switch to open position, saidspring being characterized by a substantially constant load factorwithin a range of deflection corresponding to said spacing range.

5. In a motor starter device of the type having a movable switch,resilient means operatively engaging said switch to maintain it inclosed position, and a shaftmounted speed-responsive member operativelyconnected to said resilient means to selectively disengage said meansfrom said switch, said member being axially spaced from said switchwithin predetermined dimensional limits defining a spacing range; theimprovement consisting of a conical disc spring engaging said switch inopposition to said resilient means to yieldably urge said switch to openposition, said spring being characterized by a substantially constantload factor with a range of deflection corresponding to said spacingrange, and force-compounding fingers integral with said spring andactuated by said resilient means engaging said spring.

6. A method of reducing variations in the operating speed ofcentrigually-actuated starter switches for motors resulting frommanufacturing variations in the dimensional spacing between thespring-loaded stationary and spring-loaded rotary units of suchswitches, comprising the steps of impressing a constant substantiallyuniform load on the spring of said rotary unit while simultaneouslydeflecting the spring of said stationary unit through a dimensionalrange corresponding to the range of the aforesaid manufacturingvariations in the dimensional spacing between the stationary and rotaryunits.

7. A method of reducing variations in the operating speed ofcentrifugally-actuated starter switches for motors resulting fromvariations in the dimensional spacing between the opposed spring-loadedstationary and springloaded rotary units of said switches, comprisingthe steps of impressing a load on the spring of said rotary unit, andmaintaining said load uniform while deflecting the spring of saidstationary unit in a deflection range corresponding to the range of theaforesaid vairations in dimensional spacing.

References Cited in the file of this patent UNITED STATES PATENTS1,926,339 Kindl et al Sept. 12, 1933 2,353,314 Lee July 11, 19442,583,590 Osterhas Jan. 29, 1952 2,616,682 Greenhut Nov. 4, 19522,623,961 Holstein Dec. 30, 1952 FOREIGN PATENTS 22,537 Great BritainOct. 26, 1898

